联合气枪源和背景噪声源反演郯庐断裂带南端高精度地壳波速结构

The Tanlu fault zone is the main active strike-slip fault zone in east China and plays an important role in understanding the tectonic evolution of east China. The southern tip of Tanlu fault is located in the conjunction area of three main active blocks, the North China Block, the lower Yangzte Block and Qinling-Dabie orogenic belt. As revealed by many studies, this conjunction area has endured complex tectonic evolutions, such as collision of North China and South China bocks, subduction and subsequent uplift of crust rocks, as well as the strike-slip and extension of south Tanlu fault segment. However, the details of those evolutions are still under debates. A high resolution tomography image of the complex crustal and upper mantle structure will help us to better understand the evolution history. .Many studies have been conducted to investigate the lithosphere structure there, however, due to the limited seismicity in the stable Yangzte craton and sparsely distributed stations, the resolution of velocity structure image, especially in the shallow crust, still need to be improved..In Oct 2015, in order to explore the ability on imaging crust velocity using active airgun source, we conduct an experiment along the Yangzte River using offshore airgun source. About 5000 airgun shots are excited along a 300 km long source profile, most of those shots are taken at 20 evenly spaced positions, resulting about 300 shots at one single fixed position. 700 portable seismic stations are organized into 11 linear receiver profiles to receive these signals. Preliminary results show that after 300 time stack, the body wave signals can be clearly observed at least 200 km far away. Those airgun shots greatly improve source distributions and provide one optimal source-receiver configurations to perform travel time tomography with supports of dense receivers. Furthermore, there exist over 100 permanent stations which have continuous records since 2008, shot period surface wave can be extracted from ambient noise cross correlations using those stations. The path coverages of both body wave and surface wave are dense in the conjunction of three main blocks, thus provide us a great opportunity to imaging the crustal structure using joint inversion. The expected resolution will be higher than the previous studies with contribution of body wave on high resolution of small scale characters and better constraints on shallow crustal shear velocity contributed by surface wave..In this project, we propose to use the joint inversion of body wave travel time and dispersion of surface wave to investigate the crust velocity structure in the southern tip of Tanlu fault. The body waves are generated by the airgun source while surface waves are extracted from cross correlation of ambient noise. Benefitting from the optimal source-receiver configuration and complementary properties of the body and surface waves, one high resolution three dimensional velocity structure image of this area could be obtained. .The project is the first time we try to investigate the crustal velocity structure using both the active airgun source and passive ambient noise. The expected results will delimitate the difference between those three active blocks and improve our knowledge on the geometry boundaries and possible material exchange channel between them. By analyzing the results, we could better understand the history and status of the tectonic evolution in this area and contribute to general interaction style of active blocks as well as its geodynamic mechanism.

郯庐断裂带南端位于华北地块、扬子地块及秦岭-大别造山带三个构造单元的交汇区，历史上经历了大陆地块的碰撞、俯冲和折返等强烈构造运动，形成了复杂的深浅部构造。受限于原有天然地震及台站分布，该地区尚缺少高精度浅部地壳结构图像，限制了对其构造模式的深入研究。2015年，我们沿长江安徽段进行了气枪源流动激发实验，气枪源沿300公里长水域进行了约5000次激发，并为近千个流动地震台所记录，在该地区形成了密集的观测覆盖。本项目拟利用气枪激发的体波信号，结合该区100余个固定地震台噪声互相关函数中的面波信号，对地块交汇区地壳波速结构进行体波和面波联合反演。本项目是融合气枪源和噪声源探测地壳结构的首次尝试，可获得郯庐断裂带南端块体交汇区高精度的地壳三维波速结构。研究结果有助于刻画地块间速度差异及浅部边界形态，对于了解该地区大陆地块间的相互作用和构造成因具有重要意义。

郯庐断裂带南端位于华北地块、扬子地块及秦岭-大别造山带三个构造单元的交汇区，经历了多次构造运动，形成了复杂的深浅部构造，并构成了长江中下游重要的成矿带。我们利用2015年在长江安徽段开展的气枪主动源流动激发数据，结合周边的流动台站、固定台站，利用主动源体波、面波数据，噪声互相关函数、近震走时等多种数据，在研究区开展了成像研究。主要结果为：1.利用主动源的体波走时，结合近震走时数据，通过近震走时成像方法，研究了该地区地壳的三维速度结构。结果较好的显示了断裂带南段东西部的速度差异以及成矿带波速与周边介质的差异。2.利用气枪主动源中的高频瑞利面波信号，反演了沿长江的浅部1公里内的横波速度结构，结果较好的显示了浅部横波速度的高速异常分布与四个矿集区的对应关系。3.利用固定台站的噪声互相关开展了大尺度横波速度结构研究，获取了40公里深度内的横波速度结构，结果显示了下地壳-上地幔顶部存在V形高速异常带，可能与上地幔物质上涌有关。4.综合多种体波和面波信号，开展了联合反演研究，在多源、多种数据的采样空间内，提升了相关速度结构的成像精度。同时项目也对噪声互相关函数中的多种信号开展了分析，为稳定、可靠的频散曲线提取提供了支撑。通过项目执行，探索了主动震源、天然地震、背景噪声等多种震源在研究地壳结构方面的联合分析方法，通过多种联合反演方式实现了数据的融合分析，获取了该地区较高精度的地壳波速三维速度结构。所得的结果表明大容量气枪震源的流动激发模式可为地壳结构成像提供主动探测数据，相关结果也可提升对研究区地壳至上地幔顶部结构的认识，并有助于分析郯庐断裂带南段的深部构造成因以及该地区金属成矿带的成矿机理。